[Repeater-Builder] Deviation Limiting/Linearity

27 Jul 2004 15:45:02 -0000

 
Actually, I have two problems so it might take two threads.  (Long-winded too...no less)
 
Repeater:  MSR2000
Controller:  CAT250  interfaced via Squelch Gate card.
 
I have been chasing my tail on two issues.  One is deviation limiting and the other is deviation linearity.  It seems that if the deviation is not linear when setting up IDC/Repeater Level and the deviation limiting in the receiver, if has an adverse effect on the DTMF decoding in the VA3TO interface that I use on Echolink. IRLP and WIRES decoders work nearly flawlessly with the same audio.  I found that the 147 side of the keyboard would not perform well for a lot of DTMF sources if the deviation didn't track pretty closely from 0 to 3 K or so.  We check it by putting in a 1K tone @ 1K deviation, 2K deviation and 3K deviation, etc.  There seems to be a couple of schools of thought.  One, adjust IDC/Repeater level so it tracks 1:1, e.g., 1=1, 2=2, 3=3, 4=4, etc.  The other is to check the deviation with no input and adjust the deviation to compensate for noise in the exciter, e.g., 1=1.2, 2=2.2, 3=3.2, etc. 
 
We have done it both ways.  It's seems pretty tough to get the linearity AND the deviation limiter in the receiver aligned so that the deviaton is linear and limited to about 5K and see a nice undistorted audio waveform.   After we had done an alignment and was fairly satisfied with the results, a month or so later, we began to have what appeared to be an intermod or spur problem which would break squelch and key the repeater.  We thought SURE it was a noisy rig somewhere or the TV Channel 11 spur we found a few months ago.  We chased it around several days.  We re-soldered suspected back plane pins on the R1 Audio and though we fixed it.  Then it comes back again.  Changed out the R1 Audio card and it worked for a day or two, and it's back again!  Thought it was the Squelch Gate card and checked every solder joint a few times.  Used a scope to look at everything.  This morning at about 4 AM, I put my adjusting tool on the L201 deviation limiting adjustment in the receiver and cracked it a little CW (higher deviation allowed) and the problem went away.  I could see the audio noise waveform change on the scope at the output of the receiver.  I can easily find the ragged edge so I know it's this adjustment.  I even put the suspected R1 Audio card back in and it works good.  However, 147 won't work on K7IOUs Kenwood (again) now....because the deviation is no longer linear.
 
By the way, the repeater still sounds great even if the deviaton is not linear...so long as the limiter is working.
 
Can any of you "gurus" with lots of experience give me a hint on the alignment? Or should I throw away the VA3TO interface?  Skipp, I have read your alignment procedure a hundred times, and it makes sense on paper, but I'm not confident that I'm doing it right.
 
I would think that the deviation limiting would not have an effect on the linearity, but it seems that it does. 
 
de WD7F
John in Tucson
 
 


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